There are also several websites that contain highly detailed CAD models such as: http://grabcad.com/

These CAD websites however contain files in all types of formats and thus must be converted to STL using modelling software before printing.

3D Modelling Software

Free and Open source Software:

TinkerCAD: The simplest and most basic of all 3D modelling software. Accessible directly from the browser.

Blender CAD: Primarily more of a 3D sketching and modelling tool. Not recommended for precise engineering work. However, Blender CAD contains a few advanced modelling tools not available in other free software.

Google Sketch-up: The most popular free to use CAD package, it is customisable via user made apps that can be installed free. The most advanced package listed

Professional 3D Modelling Software:

IronCad is a top tier package designed specifically for 3D printing. It remains easy to use yet contains all the nessesary features. The complete package features FEA and engineering drawing tools.

3D Slicing Software

There are several software packages that help interface the model with the 3D printer. With options such as sizing of the model, position on the print surface, the percentage fill of the model, the structure and quantity of support structures and many more!

A few free to use software packages for slicing are available on the internet. The major ones we recommend are:

Makerware:

A simple and easy to use software with limited options. Gets the job done.

Far more complex than Makerware. With options to control nearly every build condition such as support structure, specific heating of build areas, simultaneous printing of different materials and many more